The present invention relates to a smoke-permeable film of fiber-reinforced regenerated cellulose coated with a layer of a thermoplastic sealable resin, in particular a vinylidene group-containing copolymer, with a smoke-permeable structure, which layer is, if necessary, anchored to the film surface with the aid of an adhesion-promoting layer. This film is suitable for the production of tubular casings, particularly sausage casings, with a sealed or sewn seam running along the longitudinal axis. The invention further relates to a process for producing the film and to a tubular casing, in particular a sausage casing, produced from the film and having a sealed or sewn seam running along the longitudinal axis.
U.S. Pat. No. 4,396,039 discloses a tubular casing, particularly a sausage casing, which is produced from a film web of the kind described above by shaping the film web into a tube and gluing the overlapped edge regions.
Apart from a great number of other possibilities, the adhesive layer described in the published U.S. patent can, for example, also be a sealable layer of a thermoplastic, vinylidene group-containing polymer which is anchored on the surface of the film web by means of a water-insoluble adhesion-promoting resin. Since the known casing is particularly intended for sausages, which are to be smoked, emulsion type sausages and dry sausages, the adhesive layer, which forms a water- and smoke-impermeable film, can only be applied to the seam zones. The publication proposes various methods of imparting satisfactory permeability to smoke and gas to the adhesive film present in the bonding region of the film web. It is, for example, suggested that the film be applied with interruptions or passages, using a profiled coating roller, which confers a screened structure to the screened layer. Another possibility is to perforate the applied adhesive film in a subsequent step by mechanical means, or to dissolve it away partially by chemical means. Furthermore, a number of chemical processes are proposed to impart a porous structure to the adhesive layer.
This known film web and the tubular casing produced from it comprise an adhesive layer which can only be given a porous structure with the aid of relatively complicated methods. Moreover, their permeability to smoke tends to be subject to variations. The seam region of sausage casings made of this film material has irregular coloration after smoking, and often also a structured appearance. A reduction of the seam strength at discrete points may also result, so that the seam of the tubular casing will tend to burst during stuffing. Another disadvantage arises because the known adhesive layers cannot be coated onto the whole surface of the web; this would lead to a substantial reduction of the permeability of the tubular casing to smoke and water vapor over its entire surface area.